CODEWITHSUNDEEP
numpy
John Holmes
John Holmes

Reputation: 391

How to repeat a numPy array value

I was wondering if there was a vertorized way to use two arrays and based on the value of one continuously copy the value in a second array until a new value in the first array is found and then repeat the whole process.

here is an example

   a = np.array([FALSE,TRUE,FALSE,FALSE, FALSE, TRUE, FALSE  FALSE})
   b = np.array([0,10,0,0,0,20,0,0,])
   output = array([0,10,10,10,10,20,20,20])

I can use a loop which is much too slow for this.

for i in range(len(b)):   
   if b[i-1] and not b[i]:
       b[i] = b[i-1]

Updated:

I tried this code below and increased the speed by 5x but I figure there should be a faster more elegant way of doing this. 

import numpy as np

nz= np.concatenate((np.nonzero(b)[0], [len(b)]))
np.repeat(b[np.nonzero(b)], np.diff(nz))

Upvotes: 0

Views: 352

Answers (2)

Tls Chris
Tls Chris

Reputation: 3824

This generalises to n dimensional arrays. It iterates through n-1d slices of the array.

In [26]: a=np.array([
    ...:     [ 0, 0,10, 0, 0, 0,20,30, 0, 0],
    ...:     [ 4, 0, 0, 0, 0, 0, 0, 0, 0, 0],  
    ...:     [ 0, 5, 0, 6, 0, 7, 0, 8, 0, 9]])

In [27]: bfwd=0
    ...: for ix in range(a.shape[-1]):
    ...:     sel=np.s_[... ,ix]
    ...:     a[sel]=bfwd*(a[sel]==0)+a[sel]  # If a is zero use bfwd else use a
    ...:     bfwd=a[sel]

In [28]: a
Out[28]: 
array([[ 0,  0, 10, 10, 10, 10, 20, 30, 30, 30],
       [ 4,  4,  4,  4,  4,  4,  4,  4,  4,  4],
       [ 0,  5,  5,  6,  6,  7,  7,  8,  8,  9]])

HTH

Upvotes: 0

Mateen Ulhaq
Mateen Ulhaq

Reputation: 27201

Your example can be improved slightly:

b = np.array([0, 10, 0, 0, 0, 20, 0, 0])
nz = np.squeeze(np.nonzero(b))
diffs = np.empty_like(nz)
diffs[:-1] = np.diff(nz)
diffs[-1] = b.size - nz[-1]
result = np.repeat(b[nz], diffs)

...I'm not sure I understand the application though.

It might be wise to perftools your code and figure out what exactly is causing the bottleneck in your application. You might also wish to state the sizes of your arrays. (For instance, an itertools solution might be faster for fewer elements.)

Upvotes: 1

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